In the field of portable, rotary, mechanical vacuum pumps, there has been a need for some years for a small, lightweight, inexpensive pump suitable for use by refrigeration and air conditioning service personnel, a pump rated, for example, at one cubic foot of air per minute. As domestic refrigeration service is, for the most part, performed by a serviceman in the customer's home, portability and light weight are important considerations. As profit is determined by efficiency of the serviceman and his tools, a vacuum pump which will quickly draw down a refrigeration system to permit recharging, will reduce the serviceman's time on the job. Light weight will reduce his fatigue and therefor improve his efficiency. For the pump to be efficient and draw down quickly to its rated vacuum it must be free of atmospheric leakage. If the atmosphere can enter the pump, not only will it not draw down to its rated capacity, but even if the pump begins to approach its capacity, it will do so only slowly.
In constructing a rotary mechanical vacuum pump, it is conventional to construct the pump chamber from cooperating parts having flat opposed surfaces adjacent the areas of high vacuum which must be sealed against atmospheric leakage. Normal methods of sealing have been by use of resilient gaskets disposed between the surfaces, but these gaskets have a tendency to leak, generally because they are composed of a fiberous structure which provides a tortuous but nevertheless effective path for the flow of ambient air. Also, the gaskets are sometimes subject to out-gassing which in effect resembles an atmospheric leak.
In the design of rotary mechanical vacuum pumps, tolerances between moving parts become critical to prevent vacuum leaks. The use of gaskets between mating surfaces may allow some compression and therefore variation in the distance between surfaces and accordingly, may affect tolerances between the working parts within the pump chamber. Therefore, it is desirable to eliminate the use of gaskets in areas where variation in distance between mating surfaces would affect tolerances between moving parts.
To avoid these problems, it is customary to use what is known as a high vacuum grease on the mating surfaces and eliminate the gasket. This grease has a vapor pressure of 10.sup.-.sup.4 mm of mercury or less at 30.degree.C. The mating surfaces are ground or lapped to attain desired flatness and the grease spread on them and the surfaces clamped tightly together. Because of smoothness resulting from lapping, the grease tends to be squeezed out from between the surfaces leaving a dry seal which is subject to leakage. Thus, providing an inexpensive yet incompressible vacuum seal between flat mating surfaces adjacent high vacuum areas presents a difficult problem.
To reduce weight, it is desirable to utilize aluminum for the larger parts of the pump, such as the pump chamber housing. However, the softness of aluminum creates problems in wearability and because it scratches easily, mating surfaces which are to be sealed must be carefully protected prior to assembly to avoid a scratch which will permit atmospheric leakage into high vacuum areas.
I have discovered how to effect a simple and inexpensive high vacuum seal and good wearability while utilizing aluminum die cast parts.